1. Field of the Invention
The present invention relates to an image display driving controlling apparatus and its method, and more particularly, to an apparatus and method for controlling a driving voltage for an image display device, in which an optical sensor is installed between the image display device and a screen, or between the image display device and a projection lens, and the driving voltage for the image display device is feedback controlled using the output signal of the optical sensor in order to provide an optimal contrast. The present application is based on Korean Patent Application No. 2001-79714, filed Dec. 15, 2001, the contents of which are incorporated herein by reference.
2. Description of the Related Art
As shown in FIG. 1, a conventional image display apparatus is composed of a reflex image display device 1, a polarization angle adjustor 2, a polarized beam splitter 3, a projection lens 4, a focusing lens 5, a light source lamp 6 and a pair of polarization plates 7.
In the conventional image display apparatus, light emitted from the light source lamp 6 passes through the focusing lens 5, is polarized in one direction by a polarization plate 7, and then is incident upon the polarized beam splitter 3. The polarized beam splitter 3 transmits a P wave out of the incident light and reflects an S wave to change the traveling direction of light by 90°.
The S wave reflected by the polarized beam splitter 3 is incident upon the image display device 1 via the polarization angle adjustor 2. In the image display device 1, image data as shown in FIG. 2(a) is written on its pixels. A voltage applied to pixels renders the pixels in an on or off state. Light re-reflected by the image display device 1 is projected onto a screen via the polarization angle adjustor 2, the polarized beam splitter 3, another polarization plate 7 and the projection lens 4 only when the re-reflected light is in an on state.
If the image display device 1 is formed of liquid crystal, when a voltage with a uniform polarity is continuously applied to liquid crystal, the image sticking defect that liquid crystal molecules are magnetized in one direction and consequently do not operate occurs. In order to prevent this, as shown in FIG. 2(a), there is a need to drive liquid crystal with alternating current (AC) by inverting image data at predetermined time intervals. However, if inverted images are displayed as they are, pictures whose white and black have been inverted appear on the screen. Accordingly, as shown in FIG. 2(b), the polarization angle adjustor 2 is inversion driven in synchronization with the image signal of FIG. 2(a). When the image signal has been inverted, it is re-inverted by the polarization angle adjustor 2 and output as non-inverted light.
If an analog liquid crystal display device is used for the image display device 1, an image signal as shown in FIG. 3(a) is applied to the pixels of the image display device 1. In this case, the analog liquid crystal display device 1 is driven with AC in order to prevent image sticking from occurring. Here, the liquid crystal display device is driven based on the voltages of opposing base electrodes installed in the liquid crystal display device, such that the polarization angle adjustor 2 is not required.
As described above, in the event that a digital image display device is used for the image display device 1, the driving voltage for the polarization angle adjustor 2 may change from the normal state of FIG. 2(b) to the state of FIG. 2(c) as the amplitude of the driving voltage is reduced due to a deterioration of component parts or a change in temperature. This causes a degradation in the contrast of an image. As shown in FIG. 2(d), when the positive voltage and negative voltage for AC driving are asymmetrical to each other with respect to the center voltage, the contrast is degraded, flickering (blinking) appears on the screen, the quality of an image is degraded, and unpleasant pictures are displayed.
Likewise, in the event that an analog image display device is used for the image display device 1, the driving voltage of an image signal changes from the normal voltage of FIG. 3(a) to the amplitude-reduced voltage of FIG. 3(b) or the driving voltage of FIG. 3(c) in which the center voltage deviates from the voltages of opposing base electrodes. In this case, the contrast of an image is degraded, and screen blinking occurs.